As the application area of the secondary battery is extended to the electric vehicle (EV) or the energy storage system (ESS), and the like, the lithium-ion secondary battery has reached a limit situation with a relatively low weight ratio energy storage density (˜250 Wh/kg).
Among the next generation secondary battery technologies capable of achieving high energy density, lithium-sulfur secondary batteries are attracting attention as high commercialization potential compared to other technologies.
The lithium-sulfur secondary battery means a battery system using sulfur as a cathode active material and lithium metal as an anode active material.
When the lithium-sulfur secondary battery is discharged, sulfur in the cathode receives electrons and is reduced, and lithium in the anode is oxidized and ionized. The sulfur reduction reaction is a process in which a sulfur-sulfur (S—S) bond accepts two electrons and converts to a sulfur anion form, where lithium ions formed by oxidation are transferred to the cathode through an electrolyte to form a salt with the ionized sulfur.
The sulfur prior to discharge has a cyclic S8 structure and is converted into lithium polysulfide (LiSx) by a reduction reaction, where the lithium polysulfide (LiSx) is reduced in a stepwise fashion and finally becomes lithium sulfide (Li2S).
The theoretical energy density through such an electrochemical reaction is 2,500 Wh/kg, which is 10 times higher than that of lithium ion batteries.
Despite such an advantage of the lithium-sulfur secondary battery, there are many problems such as high solubility of lithium polysulfide, low lifetime characteristics and output characteristics, low electrical conductivity of sulfur, and poor stability due to the use of lithium metal.
In one example, the lithium polysulfide (LiSx) easily dissolves in the electrolyte, so that the loss of active sulfur due to repetitive charging and discharging and the resulting deterioration of cycle characteristics are considered as the biggest challenge to be solved in the lithium-sulfur secondary battery.
In order to solve the above problems, a technique of manufacturing an electrode as a porous body and then supporting sulfur within the porous bodies to inhibit dissolution possibility for the electrolyte, a technique of introducing a substance capable of adsorbing polysulfide into the electrode or a technique utilizing the hydrophilic property of polysulfide, and the like have been proposed.
However, there is still a need for continuous research on the lithium-sulfur secondary battery having excellent electrochemical performances while effectively preventing the undesired dissolution of lithium polysulfide (LiSx)